A nonrestrictive clause is a part of the sentence that you can get rid of because it’s not integral to its meaning, whereas you have to keep a restrictive clause because it is integral to the meaning.

Restrictive Clause: I lost the ring that my husband gave me.Nonrestrictive Clause: My ring, which my husband gave me, fell off when I went on the roller coaster.

The restrictive clause gives you information that you need to make sense of the sentence. Without the restrictive clause, you’d simply be saying, “I lost the ring,” which would make most people wonder, “What ring?” So in this case “that my husband gave me,” provides a key detail.

The nonrestrictive clause gives you information that is not essential to the meaning of the sentence, which is why it is separated with commas. Even though the clause “which my husband gave me” adds information, it isn’t essential. The essential information in this sentence is that she lost her ring while on the roller coaster.

What does this have to do with using which and that?

You might have noticed that the restrictive clause above used that and the nonrestrictive clause used which.

In all cases, you must use that to introduce a restrictive clause, but you can use which or that to introduce nonrestrictive clauses. Confused? If you prefer, you can just stick to that for restrictive clauses and which for nonrestrictive clauses if it helps clarify things.

So consider whether which or that belongs in the following sentence:

“My husband’s car ____ was parked across the street was stolen last night.”

The clause, “was parked across the street,” is not a necessary part of the sentence. You can use which or that.

Let’s look at another example:

“My brother found the woman ___ he was going to marry.”

If you guessed that, you’re technically correct. This was a trick question. This is a restrictive clause (“he was going to marry” is an important detail in this sentence), so based on the rules we just discussed, you’d use that. But a lot of people would argue you should use who to refer to a person instead of that.

The who vs. that debate is one for another day — next Thursday to be precise. See you then.